1. Field of the Invention
The present invention relates to a deformable curvature mirror that is capable of controlled deformation by the application of electrical voltages to the mirror and, in particular, is directed to such a mirror that is for use in an adaptive optics system having a wavefront sensor for controlling the deformation of the curvature mirror.
2. Background of the Invention
There are various adaptive optics methods and devices which include a wavefront sensor for sensing the aberrations in the wavefront of light waves and then correcting or compensating for those aberrations, such as the atmospheric aberrations that effect the viewing of stars and planets through a telescope. The existing methods and devices for sensing and measuring the wavefront include several interferometric techniques, the Shack-Hartmann wavefront sensing techniques and various other systems involving the projection of patterns of light through an optical system.
Such prior techniques and systems are typically complex and expensive, as well as having various inherent deficiencies. In addition to the deficiencies of the prior art wavefront sensors, the deformable mirrors that are controlled by those wavefront sensors for adaptive optics also include numerous deficiencies. For example, a stack actuator mirror is comprised of a multiplicity of push rods engaging the back of a flexible mirror and the extension-retraction of each push rod is usually controlled by a Shack-Hartmann wavefront sensor. The Shack-Hartman wavefront sensor measures local slopes of the wavefront and these slopes are fitted with a wavefront reconstructor which in turn generates a continuous surface, matching all the slopes. This type fitting is blind to hysterisis effects in the actuators thereby causing a waffle pattern to appear on the mirror surface. The push rods tend to produce a deformation which is nearly a straight line on the mirror surface between each pair of adjacent push rods that results in large fitting errors When a small number of actuators are used. Moreover, the number of push rods and, therefore, the closeness of the push rods is physically limited, as well as the length of their travel, whereby the accuracy and degree of optical correction that can be applied by the stack actuator type mirror is limited. Since all actuators have the same travel and are attached to a rigid reference surface, the mirror has the same stroke for all modes, i.e. low order focus has the same stroke as the highest mode produced by every other actuator being turned on and off. For correcting the aberrations originating in the atmosphere, this range of stroke at the highest modes is not necessary, whereby the corrections may not be accurate for small errors.
One deformable mirror that overcomes many of these problems uses electro-restrictive material to controllably deform the mirror. Voltages are selectively applied to electrodes to deform the mirror. However, voltage sources that are capable of providing bipolar voltages (both positive and negative) are expensive. In addition, the electro-restrictive materials have a polarity, meaning that they respond better to voltage of one polarity. When the voltage of the reverse polarity is applied, the electro-restrictive material performs more poorly and may even be damaged so as to lose its electro-restrictive properties. Therefore, it is desirable to provide a deformable mirror that uses unipolar voltage sources (either positive or negative) to control the deformation of the mirror.